Astronomers have used the gravitational warping of light, predicted by Einstein nearly a century ago, to measure the mass of a distant star for the first time.

EINSTEIN KEY POINTS

Key points

The effect of gravity on light can be used to measure the mass of objects in space

Einstein said it would be impossible to observe this phenomenon with distant stars

Study provides clues about the fate of our own Sun

The team, led by Kailash Sahu of the Space Telescope Science Institute in Baltimore, measured the mass of a white dwarf star called Stein 2051 B as it passed in front of another more distant star — an event Einstein thought would be impossible to observe.

The findings, publishing in today's edition of Science, will help us understand more about these small dense stars and the ultimate fate of our Sun, which will become a white dwarf star when it burns out.

Dr Sahu relied on Einstein's idea that the gravity of an object can bend and magnify rays of light.

According to Einstein's general theory of relativity, an object passing in front of another bright object would bend the light of the more distant object and cause it to appear to move from its original position.

In this "gravitational lensing", the distance apparently travelled by the background object depends on the mass of the object in front, which is curving the light.

While gravitational lensing has been used by astronomers to study the galactic bulge in our Milky Way and other galaxies such as Andromeda and the Magellanic Clouds, this is the first time it has been used to study distant stars.

'No hope' of observing, said Einstein

At the time Einstein came up with the idea of gravitational lensing, he did not think it would be possible to use it to observe distant stars.

In a 1936 Science paper he wrote there would be "no hope" of observing it directly because stars were too far apart.

"He thought it would be practically a very difficult experiment to do, especially given the technology of the day," said Geraint Lewis, a cosmologist who works on gravitational lensing at the University of Sydney.

In Einstein's time, telescopes were much less advanced so the chance of seeing one star pass in front of another — and being able to observe the lensing effects — was much lower.

"Technology has now caught up," said Professor Lewis. "You can actually do the experiments that Einstein didn't think were possible."

Even though these passings are rare, said Professor Lewis, these days we can look at a lot of stars in one go.

"It's a one in a million chance, so what you do is you look at a million stars at a time," Professor Lewis said.

Star catalogue helps search for candidates

For their study Dr Sahu and team used data from star catalogues to project the positions of around 5,000 stars and look for any cases where they were likely to pass in front of fainter background stars.

Once they discovered that Stein 2051 B — 18 light-years from Earth — was a perfect candidate, they were able to use the Hubble Space Telescope to observe the event.

"It's hard work to do this kind of observation and separate the starlight from the white dwarf from the background star, and find the positions accurately. So it's pretty cool stuff," Professor Lewis said.

"What these guys have done for the first time is looked at how the motion of a star changes as one passes in front of the other and used that to measure mass."

How gravitational lensing affects the observed position of the background star

How gravitational lensing affects the observed position of the background star

Supplied: NASA, ESA and A. Feild (STScl)

Star reveals itself to be a 'bog standard' white dwarf

Dr Sahu and his team found the mass of the white dwarf agreed with the Chandrasekhar limit, a threshold proposed by Indian astrophysicist Subrahmanyan Chandrasekhar that determines whether or not a white dwarf will collapse into a supernova.

"When I found that the mass of the white dwarf is precisely in accordance with what Chandrasekhar had predicted in 1930 in his Nobel-prize winning theory of white dwarfs, I fell off my chair," Dr Sahu said.